structures. And when coastal aquifers are overused, salt water from the ocean may enter the
aquifer, contaminating the aquifer and making it less useful for drinking and irrigation.
Most land areas have some kind of aquifer beneath them. Aquifers can occur at different
depths and different geographic locations. The closer aquifers are to the surface, the more
likely they will be used by humans. However, closeness to the surface also increases the
probability that the aquifers could be contaminated by surface pollution that seeps through
the porous rock along with the water. Aquifers are usually not open spaces like caverns
or swimming pools, but instead are porous rock and sediment. The spaces between the
sediments or rock particles are filled in with water. Wet sand at the beach is a good model
for the consistency of most aquifers.
The Ogallala Aquifer is one of the world’s largest aquifers, and is a particularly important
source of freshwater in the United States. It lies beneath eight United States states — South
Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas. It
ranges from less than a meter deep to hundreds of meters deep and covers about 440,000
square kilometers! It is widely used by people for municipal and agricultural needs. However,
its rate of replenishment is only about 10% of the rate at which it is being used. In other
words, for every 100 liters of water that people withdraw from the aquifer, only 10 liters are
being naturally replaced by precipitation. This overuse of the aquifer has created political
controversies and disputes in those areas that depend on the aquifer.
Figure 13.21: Agricultural irrigation often depends on water from aquifers. ( 21 )